1. Field of the Invention
This invention relates to a projection type display apparatus for projecting an enlarged composite image of a plurality of images formed upon respective light valves, and particularly to the performance improvement of the apparatus for projecting a composite image composed of images formed on a plurality of light valves.
2. Description of the Related Arts
FIG. 4 of the accompanying drawings is a schematic view showing a structure of an optical system of a conventional projection type display apparatus. In FIG. 4, a reference numeral 1 designates a light source; 2, a lamp; 3, a reflecting mirror; 4, a luminous flux irradiated from the light source 1; 5R, 5G, 5B, liquid crystal light valves; 15R, 15G, 15B, image forming faces of the liquid crystal light valves; 6R, 6G, 6B, condenser lenses; 7R, 7B, dichroic mirrors for color separation; 8B, 8G, dichroic mirrors for color composition; 9, 10, mirrors; 11, a projection lens system; and 12, a screen.
The operation of the conventional display apparatus will now be explained.
The luminous flux 4, having been irradiated from the white color light source lamp 2, is reflected by the reflecting mirror 3. The dichroic mirror 7R for color separation reflects a red light and allows the blue and green lights to pass through. The red light separated by the dichroic mirror 7R is then reflected by the mirror 9 and is incident on the liquid crystal light valve 5R. Meanwhile, the dichroic mirror 7B reflects a blue light, and allows a green light to pass through.
Accordingly, the blue, green and red lights are projected onto the liquid crystal light valves 5B, 5G and 5R, respectively. Upon each of the image forming faces 15G, 15B, 15R of the liquid crystal light valves 5G, 5B, 5R is formed an image of respective primary color, i.e., green, blue and red by means of non-illustrated external circuits, and the emitted luminous flux is subjected to transmission modulation inside of the light valves.
The beam emitted from the light valves 5G, 5B, 5R are incident on the projection lens system 11 as a composite luminous flux 13 by way of the dichroic mirror 8B for reflecting blue light, the dichroic mirror 8G for reflecting green light, and the reflecting mirror 10. This composite luminous flux 13 is converged into an image upon the screen 12 by means of the projection lens system 11, and an enlarged color image is finally projected for audience appreciation.
Here, the size of the liquid crystal light valves 5R, 5G, 5B are equal and disposed at the same distance from the projection lens system 11. As a result, an image of each primary color is formed in an area on the screen 12 approximately equal in size. Moreover, when a zoom lens is used as the projection lens system 11, the projection magnification factor can be varied and the size of the display area on the screen 12 can be freely changed. The condenser lenses 6R, 6G, 6B are utilized for letting the red, green, blue lights be incident on the projection lens system 11 with high efficiency.
Next, the operation and structure of the liquid crystal light valve 5 is explained with reference to FIG. 5.
The liquid crystal 20 is sandwiched between two glass plates 21 and 22, and this liquid crystal with the glass plates is further sandwiched between the polarizing plates 23 and 24. When no voltage is applied, i.e., V=0 as shown in FIG. 5a, a polarizing direction of a linearly polarized beam 4a which is transmitted through the polarizing plate 23 on the incoming side is rotated at 90 degrees by an optical rotatory power of the liquid crystal when transmitting through the liquid crystal 20. As a consequence, the linear polarized beam 4a transmits through the outgoing side polarizing plate 24, the polarization axis of which is arranged to be orthogonal to that of the incoming side polarizing plate 23. On a contrary, when the voltage, more than the threshold voltage Vth, is applied as shown in FIG. 5b, the optical rotatory power of the liquid crystal diminishes, the intensity of light passing through the outgoing side polarizing plate 24 is decreased in proportion as the increase of the voltage. By utilizing such a control action of the transmissivity and the structure of the electrodes (not shown) in two-dimensional array, image display elements can be formed in two-dimensional matrix. In the liquid crystal light valve 5 of FIG. 5, the boundary surface between the liquid crystal 20 and the outgoing side glass plate 22 constitutes the image forming face 15. The above explanation of the liquid crystal 20 is based on an example in which a TN (twisted nematic) liquid crystal having the optical rotatory angle of 90 degrees is used in normally white mode. As is widely known, although there are several modified applications other than the above example, the explanation of them is omitted here because they have no relevancy to the present invention.
A description will now refers to the projection lens system 11.
The projection lens system 11 used in the conventional projection type display apparatus consists of a combination of plural lenses, so as to obtain an optimum projection image with various types of aberrations suppressed as much as possible. However, since the wavelength of each of the three primary-colors, red, green, and blue, is distributed in a wide band range between 400 nm and 700 nm, the magnification of the projection lens system for forming an image is disadvantageously different for every primary color (chromatic aberration of magnification).
Referring to FIG. 6, the above-mentioned drawbacks in the prior art are explained hereunder. FIG. 6a shows the projection lens system 11 composed of a positive lens 11a, a negative lens 11b, and a diaphragm 11c. Assuming that the composite beam 13, composed of luminous fluxes emanated from pixels positioned at the same location in the image of respective liquid crystal light valves 5R, 5G, 5B, is incident on the projection lens system 11. In FIG. 6, for simplification, the green light is indicated as a principal ray and other lights, or blue and red lights, are treated as subordinate rays. Generally, the shorter a wavelength becomes, the higher the refractive index of a glass increases. Therefore the refractive index of the glass becomes highest in blue and lowest in red. Due to such a characteristic, as illustrated in FIG. 6a, the image produced on the screen is maximum in blue and minimum in red. FIG. 6b shows a state in which an image is formed by a pixel onto the screen 12. As described above, the image forming magnification factor becomes small in the order of blue, green and red (blue&gt;green&gt;red). Eventually, images of the blue pixels (a rectangular with right ascending lines) are formed outside relative to images of the green pixels (a rectangular with blank), whereas the images of the red pixels (a rectangular with right descending lines) are formed inside relative the green images. At the screen center, however, since the influence of the magnification chromatic aberration is small, the pixels of the three primary colors converge.
As seen from FIG. 6a, the above description applies to the case in which the positive lens 11a and the negative lens 11b are positioned one by one from the light valve side. If the arrangement order of the lenses is reversed, the relationship of the magnification chromatic aberration is inverted to an order of red&gt;green&gt;blue.
In the liquid crystal light valve, since the pixel pitch is set with high precision when manufacturing the liquid crystal plate, it is difficult to freely change the size of the display area when displaying the image. To solve this problem, a disparity (a disparity of the pixels) due to the chromatic aberration of magnification is accurately matched by adjusting the projection magnification factor of the image of the indicated primary-color by the projection optical system. As a numerical example, the size of the disparity of the three primary-colors due to the magnification chromatic aberration of the projection lens must be reduced by as little as one-tenth of one pixel. In the display of 500.times.500 pixels, for example, the disparity of the magnification ratio in every primary color must be suppressed approximately to a degree of +/-1/2500 (+/-0.04%) for securing the allowable disparity at the corners of the screen. As a result, the number of lenses which are required in the projection lens system and the cost of the projection display apparatus are increased.